The spread of the Internet and an increase in the capacity of data to be transmitted have led to a massive increase in traffic. This tendency has led to energetic development of technology for increasing the capacity of optical wavelength multiplex communication systems.
When devices constituting a communication system are connected to each other through a plurality of optical fibers, a method, wherein a bundle of a plurality of optical fibers is provided and is used for connection between these devices, is in many cases more convenient than a method wherein the devices are connected to each other by separately drawing these optical fibers one by one. In this case, a tape optical fiber cord (hereinafter often referred to as “tape fiber”), in which a plurality of optical fibers are juxtaposed and are then covered to bring the optical fiber into a tape, is used.
FIG. 1 shows a fiber cord with a half pitch fiber array which is an example of this type of conventional tape fibers. The term “half pitch” as used herein means that optical fibers are arranged at a spacing (around 125 μm) of substantially the half of general optical fiber arrangement spacing (typically around 250 μm) as full pitch in fiber cords. In the example of the fiber cord shown in FIG. 1, one end of the fiber cord 111 with a half pitch fiber array is fixed to a half pitch fiber array 112 with a pitch of 127 μm, and the fiber cord comprises four tapes of a first upper tape 1131, a second upper tape 1132, a first lower tape 1133, and a second lower tape 1134 which are arranged in two stages. In these tapes (first upper tape 1131, second upper tape 1132, first lower tape 1133, and second lower tape 1134), optical fibers are arranged at a full pitch.
FIG. 2 shows the sectional structure of this fiber cord with a half pitch fiber array. It is assumed that each of the four tapes of the first upper tape 1131, the second upper tape 1132, the first lower tape 1133, and the second lower tape 1134 comprises a 12-core tape core in which 12 optical fibers 115 are arranged. In this example, the pitch of the optical fibers 115 in each of the tapes 1131 to 1134 is 250 μm.
In one end of the fiber cord 111 with a half pitch fiber array, the optical fibers constituting the four 12-core tape core are separated one by one into 48 optical fibers 1151 to 11548. Optical connectors 1171 to 11748 for input/output of optical signals are connected to the respective ends of the optical fibers 1151 to 11548.
FIG. 3 shows the arrangement of optical fibers in a fiber aligning section located on the side of an optical waveguide (not shown). On the inlet side of the half pitch fiber array 112 shown in FIG. 1, 48 optical fibers in total in the fiber cord 111 with a half pitch fiber array shown in FIG. 1 are arranged as shown in FIG. 2. In the fiber aligning section 121 located on the opposite side of the inlet side, the optical fibers are arranged in one stage at a pitch of 127 μm. Thus, on the optical waveguide side, the arrangement structure is such that, in consideration of a demand for multichannel and a reduction in size of devices, the pitch of output side waveguides has been reduced to the half of the pitch, that is, the half pitch, of the optical fibers 115 on the tapes 1131 to 1134 side. In the fiber cord 111 with a half pitch fiber array in this example, a two-stage arrangement structure is adopted as shown in FIG. 1 or 2. Therefore, at a portion before the fiber aligning section 121, the optical fibers 115 are taken out one by one from the upper stage and the lower stage by turns and are aligned in the fiber aligning section 121.
FIG. 4 shows the rearrangement of the optical fibers in the fiber array. The numerals from “1” to “48” in this drawing (FIG. 4) and FIGS. 1 and 3 designate serial numbers of the optical fibers. Even when the numbers of the optical fibers 1151 to 11548 are regularly permuted one by one in an ascending order on the fiber aligning section 121 side, the optical fibers 1151 to 11548 are distributed to and arranged in the four tapes 1131 to 1134. As a result, on the other end side of the fiber cord 111 with a half pitch fiber array, as shown in FIG. 1, the optical connectors 1171 to 11748 are arranged in intermittent permutation. Therefore, workers responsible for connection of the optical connectors 1171 to 11748 should perform connection work while selecting corresponding optical fibers 115, for example, from the first tape 1131 and the third tape 1133. This makes the work troublesome, requires a prolonged work time, and, at the same time, increases the probability of the incidence of mistakes in the work.
FIG. 5 shows a conventional fiber cord with a half pitch fiber array which has overcome the above problems. In FIGS. 5 and 1, like parts are identified with the same reference numerals, and the overlapped explanation thereof will be omitted. In this fiber cord 131 with a half pitch fiber array, a branching case 132 is provided between the half pitch fiber array 112 and the optical connectors 1171 to 11748.
In the fiber aligning section 121 as the end on the optical waveguide (not shown) side in the half pitch fiber array 112, the optical fibers 115 are regularly arranged one by one in an ascending order as shown in FIG. 3. Therefore, the arrangement of the optical fibers 115 between the half pitch fiber array 112 and the branching case 132 is the same as shown in FIG. 3. In the branching case 132, the optical fibers 115 arranged in this way are rearranged. Specifically, in the first tape 1331, optical fibers “1” to “12” in the optical fibers 115 are arranged in that order, and, in the second tape 1332, optical fibers “13” to “24” in the optical fibers 115 are arranged in that order. Likewise, in the third tape 1333, optical fibers “25” to “36” in the optical fibers 115 are arranged in that order, and, in the fourth tape 1334, optical fibers “37” to “48” in the optical fibers 115 are arranged in that order. Therefore, workers can correctly mount the optical connectors 1171 to 11748 in a short time.
FIG. 6 shows wiring in the interior of the branching case. Here for simplification of the drawing, only optical fibers “1” to “24” in the optical fibers 115 are shown. The first upper tape 1131 shown in FIG. 5 and the first lower tape 1133 not shown in FIG. 5 are connected to the left side (half pitch fiber array 112 side) in the drawing of the branching case 132. On the other hand, the first upper tape 1331 and the first lower tape 1333 both shown in FIG. 5 are connected to the right side (optical connectors 1171 to 11724 side) in the drawing of the branching case 132. For clarification of the drawing, the first upper tapes 1131, 1331 are indicated by a solid line, and the first lower tapes 1133, 1333 are indicated by a dotted line. In the interior of the branching case 132, optical fibers are connected so as to realize the change in arrangement of left and right optical fibers. In this way, when the branching case 132 shown in FIG. 6 is used, the work of connection between the tape optical fiber cord 131 with an optical fiber array and the optical connectors 1171 to 11748 can be smoothly carried out without difficulty.
The conventional fiber cord 111 with a half pitch fiber array shown in FIG. 5, however, has a problem that, since one end thereof is fixed to the branching case 132, microbends often occur. Here the term “microbends” means that the application of uneven force to the optical fibers 115 causes bends having a radius which is large and is not negligible as compared with the wavelength of light propagated through the core. When microbends occur, light propagated through the core is leaked out from the microbends to the outside of the optical fiber, leading to transmission loss. Due to this unfavorable phenomenon, a difference in characteristics occurs among the optical fibers 115 constituting the tape optical fiber cord with an optical fiber array.
For example, Japanese Patent Laid-Open No. 148480/1994 discloses a method for preventing the occurrence of microbends. In this method, before a molded product of a tape optical fiber core is wound, the tape is coated with a silicone oil dissolved in a volatile solvent to increase the lubricity of the tape, whereby the wound state of the tape is improved to avoid the application of uneven force to the optical fiber core and thus to prevent the occurrence of microbends. The invention solves the problem of the occurrence of microbends during the production of the tape optical fiber cord and, at the same time, solves the problem of the occurrence of microbends during the use of the produced tape optical fiber cord with an optical fiber array.
FIG. 7 illustrates microbends which occur during the use of the tape optical fiber cord with an optical fiber array using the branching case shown in FIG. 5. The branching case 132 is connected to the tape optical fiber cord 131 with an optical fiber array in its end remote from the half pitch fiber array 112, and the portion between both ends of the tape optical fiber cord is partially wound. In the where the cable is disposed between communication devices not shown, it is common practice to use a tape optical fiber cord 131, with an optical fiber array, having a length which is somewhat larger than necessary, for example, in consideration of relocation of the devices in the future. In this case, the cord portion between both ends of the tape optical fiber cord is partially wound by a predetermined number of times to regulate the whole length.
In FIG. 7, for easy understanding of the explanation, as with FIG. 6, the first upper tape 1131 is indicated by a solid line, and the first lower tape 1133 is indicated by a dotted line. The half pitch fiber array 112 is fixed to the left end in the drawing of the tape optical fiber cord 131 with an optical fiber array, and the branching case 132 is fixed to the right end of the tape optical fiber cord 131 with an optical fiber array. The lengths of the first upper tape 1131 and the first lower tape 1133 between the half pitch fiber array 112 and the branching case 132 are equal to each other.
Assuming that the cord portion between both ends of the tape optical fiber cord 131 with an optical fiber array is partially wound in a given direction once or by a plurality of times, that, as shown in the drawing, in the ring-like bent portion, the first upper tape 1131 is placed inside the first lower tape 1133, and that the ring-like bent portion is round, the radius R1 of the circle in the first upper tape 1131 is smaller than the radius R2 of the circle in the first lower tape 1133. Therefore, the length of the circumference in the first upper tape 1131 is shorter than the length of the circumference in the first lower tape 1133. Since the lengths of the first upper tape 1131 and the first lower tape 1133 between the half pitch fiber array 112 and the branching case 132 are equal to each other, in the unbent straight portion, the first upper tape 1131 is longer by the difference in length in the ring-like bent portion than the first lower tape 1133.
This length difference can be determined by equation (1):ΔL=2πtm  (1)wherein
ΔL represents length difference;
t represents spacing between the core of the first upper tape 1131 and the core of the first lower tape 1133 in the thicknesswise direction in the tape optical fiber cord 131 with an optical fiber array; and
m represents the number of times of winding in the ring-like bent portion.
The optical fibers undergo local stress under positional restriction, for example, by a covering material at several sites between both ends of the tape to absorb the excess of length of the first upper tape 1131. As a result, the optical fibers are bent, and microbends 134 occur. A fluctuation in environment temperature also accelerates the occurrence of microbends.
In the above explanation, although a two-stage, two-column tape optical fiber cord 131 with an optical fiber array has been taken as an example, any multistage tape optical fiber cord with an optical fiber array has a possibility of posing the same problem of occurrence of microbends as described above. Further, in the above explanation, although the use of the branching case 132 for changing the arrangement of the optical fibers 115 has been taken as an example, the fixation of the optical fiber cord in its end remote from the optical waveguide by any means poses the same problem as described above.